1. Field of the Invention
The invention relates to a process for manufacturing a toughened and/or bent, infrared-reflecting sheet of glass made of sodalime silicate glass, in which on at least one side of a transparent glass base a silver coating and on its side facing away from the glass base at least one anti-reflection coating made of metal oxide are applied and a thermal toughening and/or bending process at a temperature of 580.degree. C. to 680.degree. C., preferably 600.degree. C. to 650.degree. C. is carried out, a sheet of glass produced according to this and its application.
2. Description of the Prior Art
Thin silver coatings are characterised by high light transmission in conjunction with high infrared-reflection and high electrical conductivity. For this reason, they have found many applications, for example for improving the thermal insulation of window panes and as a heated coating for windscreens in motor vehicles. The selective filtering properties of silver coatings can be further improved if on the side of the silver coating facing away from the glass base an anti-reflection coating made of a die-electric material is arranged, with an index of refraction .gtoreq.1.7, for example a metal oxide coating. Furthermore, between the glass base and the silver coating a further di-electric coating can be provided, which acts as a bonding agent and if designed as a quarter-wavelength coating provides an additional anti-reflection effect. It also lies within the scope of the generic measures that the silver coating is embedded on both sides in each case one or several metal oxide coatings. Furthermore, between the glass base and the silver coating and the metal oxide coatings and the silver coating, metal, metal oxide or metal alloy coating, for example made of chrome, nickel, titanium, chromium nickel alloys, can be arranged, in order for example to improve the filtering properties, the adhesive strength and the resistance to corrosion (see DE-OS No. 21 44 242, DE-OS No. 33 07 661, DE-PS No. 33 16 548, EP-PS No. 35 906 and EP-OS No. 104 870).
In many cases of application, it is necessary to thermally toughen the glass base, for example to increase the mechanical stability, to avoid heat cracks and to reduce the risk of injury in the event of the glass sheet breaking. In order to create the thermal toughening, the sheets used almost exclusively in the applications referred to made of sodalime silicate glass are heated up quickly in air to a temperature above the transformation temperature of the glass and then tempered. The temperatures required for the toughening process lie in the region of 580.degree. C. to 680.degree. C., preferably 600.degree. C. and 650.degree. C. The same temperature range is also required if the glass sheets which are flat after glass production are subjected to a bending process, in order to obtain bent sheets of glass. Such glass sheets are particularly used as heated windscreens and rear lights when glazing motor vehicles. In connection with this, the glass sheet provided with an electrically conducting silver coating is bonded to another sheet of glass via a polyvinylbutyral film, whereby the coating is found on the side facing the lamination film, provided with the appropriate current conductor bars in the edge area of opposite sides for the feeding of the electrical energy for heating purposes. The application of the above-named coatings has until now regularly been carried out after concluding the toughening or bending process, whereby normally the vacuum coating process is used; it is admittedly known from DE-OS No. 36 28 057 that sheets of glass can be provided with a heat resistant pure metal coating system, in which an outer metal oxide coating is atomised non-reactively. For this purpose it is however necessary to use an oxide target for applying the outer metal oxide coating, which, alongside increased expenditure in the manufacture of the target, has the disadvantage of only relatively low sputtering rates. In addition, this process is associated with a deterioration in light transmission compared with generically produced silver coating systems.
The process of carrying out the coating after the toughening or bending process, as is usual with the generic process, has various disadvantages compared with a method in which initially the coating is applied and then the toughening or bending process is carried out. Thus in the first case, only cut sizes can be coated, as it is known that toughened glass sheets cannot be cut to size. For coating technology, it is on the other hand much more beneficial to coat unit sizes, in particular the machine widths from glass produced using the float process. In the latter case, the problem of a uniform coating thickness with vacuum coatings can be solved much more easily and simply than when cut sizes with corresponding gaps between the individual sheets in the coating field are coated. In addition, conveyance of such unit sizes through the coating system is less time-consuming than when individual pieces of various dimensions have to be conveyed.
A further disadvantage lies in the fact that as a result of the high temperatures of the toughening or bending process impurities on the surface of the glass frequently are bonded so firmly to it that in the subsequent surface cleaning before carrying out the coating process they cannot be removed to the extent that is necessary for the subsequent coating process. This leads to a troublesome deterioration in the coating quality.
In the case of coating bent sheets of glass, the problems of obtaining a sufficient uniformity in the coating are of course particularly great, because the angle and distance to the coating sources change additionally as a result of the curvature of the glass sheets. In addition, the cost of vacuum coating systems for coating bent sheets of glass are considerably higher than for the coating of flat sheets of glass, as input and output locks and locks between the various coating stations must be considerably wider than when coating flat glass.
For the reasons referred to, a procedure in which flat glass, in particular in the form of unit measures, is coated and then--in particular after producing the cut sizes by separation--is toughened or bent, has considerable advantages. This procedure is however impossible with coatings of the type described at the beginning, because troublesome changes in the coating would occur as a result of the heat stressing associated with the bending or toughening process. The sheets of glass receive a matt and in some cases spotted appearance. These changes in the coating lead to incident light being scattered. The scattered light level is in this case so intensive that it for example considerably impairs vision through such a sheet of glass, if this sheet is irradiated obliquely by sunlight and an observer looks through from the other side (milky sheet effect). In such conditions, there can be a troublesome impairment of vision through the sheet if only 1% of the solar radiation is diffusely scattered. Moreover, the coating loses in part its selective filtering properties. The light transmission, the infrared reflection capacity and the electrical conductivity deteriorate.